Narrative:

We were cleared from FL410 to FL230. During the descent from cruise altitude; smoke began to come out of the overhead cockpit vents. A few seconds later; more smoke was pouring in the cockpit. We put on our oxygen masks and looked back towards the cabin. There was no smoke there. The smell was of overheated electrical/insulation. We declared an emergency and requested further descent to ZZZ which was the closest airport; about 35 NM ahead. We were given a lower altitude (14000 ft) and a heading. Later we were given a few more stepdowns; another heading; instructions to join the ILS at 5700 ft and cleared to land. We requested that the fire trucks stand by. There was now smoke in the cabin as well but it didn't look like it was getting thicker. We canceled the emergency on short final; landed and taxied over to the FBO where the fire trucks met us. I shut down the engines and turned off the batteries while the copilot deplaned our passenger. The smoke had stopped. The firemen investigated the cabin and found the area around the cabin fan to be very hot. The aircraft batteries were disconnected; they ventilated the cabin and stood by for a while. We unloaded the bags and sent our passenger on their way.

Google
 

Original NASA ASRS Text

Title: LJ35 CAPT RPTS SMOKE IN COCKPIT DURING DSCNT FOR LNDG. OXYGEN MASKS ARE DONNED AND AN EMER IS DECLARED. SITUATION STABILIZES AS THE FLT CREW LANDS AT DEST ARPT.

Narrative: WE WERE CLRED FROM FL410 TO FL230. DURING THE DSCNT FROM CRUISE ALT; SMOKE BEGAN TO COME OUT OF THE OVERHEAD COCKPIT VENTS. A FEW SECONDS LATER; MORE SMOKE WAS POURING IN THE COCKPIT. WE PUT ON OUR OXYGEN MASKS AND LOOKED BACK TOWARDS THE CABIN. THERE WAS NO SMOKE THERE. THE SMELL WAS OF OVERHEATED ELECTRICAL/INSULATION. WE DECLARED AN EMER AND REQUESTED FURTHER DSCNT TO ZZZ WHICH WAS THE CLOSEST ARPT; ABOUT 35 NM AHEAD. WE WERE GIVEN A LOWER ALT (14000 FT) AND A HDG. LATER WE WERE GIVEN A FEW MORE STEPDOWNS; ANOTHER HDG; INSTRUCTIONS TO JOIN THE ILS AT 5700 FT AND CLRED TO LAND. WE REQUESTED THAT THE FIRE TRUCKS STAND BY. THERE WAS NOW SMOKE IN THE CABIN AS WELL BUT IT DIDN'T LOOK LIKE IT WAS GETTING THICKER. WE CANCELED THE EMER ON SHORT FINAL; LANDED AND TAXIED OVER TO THE FBO WHERE THE FIRE TRUCKS MET US. I SHUT DOWN THE ENGS AND TURNED OFF THE BATTERIES WHILE THE COPLT DEPLANED OUR PAX. THE SMOKE HAD STOPPED. THE FIREMEN INVESTIGATED THE CABIN AND FOUND THE AREA AROUND THE CABIN FAN TO BE VERY HOT. THE ACFT BATTERIES WERE DISCONNECTED; THEY VENTILATED THE CABIN AND STOOD BY FOR A WHILE. WE UNLOADED THE BAGS AND SENT OUR PAX ON THEIR WAY.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.